1. Field of the Invention
This invention relates generally to a submersible motor and more particulary to a submersible motor having a primary winding constituted by a water-tight wire having an excellent dielectric breakdown characteristic, a high exterior damage resistance and an excellent water-tree-proofness. The motor, as a result of the characteristics of the primary winding has a markedly enhanced life characteristic.
2. Description of the Prior Art
The most commonly used submersible motors are those of the water-tight type which are filled with water when in service. Since water-tight type submersible motors are always used in the water, the water resistance of their insulation system, particularly the winding, is the most important factor in the determination of its reliability and life.
Many kinds of insulating wires with varying structure have been used for the windings in submersible motors. The more commonly employed wires have three insulating layers: an enamel layer (innermost layer), a main insulating layer (middle layer) and an outer protective layer (outermost layer). For the enamel layer, a polyvinyl formal polymer, a polyester polymer, an epoxy polymer or a polyamideimide polymer is used. The function of this layer is not completely understood except that it acts as a copper damage-shielding layer when the conductor is a soft copper wire. Moreover, it is empirically known that this layer prolongs the life of submersible motor windings.
As the main insulating layer polyethylene, particularly crosslinked polyethylene and polypropylene, are widely used because they excel in water resistance, are low in price and good in processability compared with other kinds of polymers. However, insulated wires with a main insulating layer composed of these polymers, when used in a submersed conditions, have the drawback that the main insulating layer is locally deteriorated by water treeing in a short period of time. In order to overcome this drawback, the use of a mixture of a crystalline polypropylene and an ethylene-propylene rubber or an ethylene-propylene terpolymer as the main insulating layer has been proposed in Japanese Patent Publication No. 33140/1970. However, the combination of the abovementioned enamel layer and this main insulating layer alone is not able to provide satisfactory characteristics.
While a crosslinked polyethylene and polypropylene are suitable from the point of view of dielectric strength and economy, they are not necessarily superior in their resistance to exterior damage. Accordingly, when these polymers are used as the outermost insulating layer, the wire surface may be scratched during winding and other operations, thereby causing the deterioration of the insulation and the breaking of the wire. In order to prevent this phenomenon, the main insulating layer is generally covered by an outer protective layer composed of a material having a high resistance to exterior damage. For example, the use of a covering of polyamide polymer is proposed in Japanese Patent Public Disclosure (Laid-Open) No. 37823/1980. Although polyamide polymers are superior in exterior damage resistance, they are inferior in water-sealing properties. Therefore, the polymers cannot sufficiently prevent water penetration into the main insulating layer composed of polyethylene or polypropylene and accordingly cannot effectively prevent the deterioration of the main insulating layer due to a water treeing phenomenon.
Thus, water-tight wires having a conventional outer protective layer composed of a polyamide polymer have had the drawback that, when used as the windings in a submersible motor, their insulation deteriorates in a short period of time and thus they cannot withstand long-term use.